Metallic components of dry cell batteries sometimes have inner surfaces which are in contact with the electrolyte and outer surfaces which are not. These metallic components may be an electrode--particularly the negative electrode or anode, but also sometimes the positive electrode or cathode--and/or a terminal for the battery.
During the electrochemical reactions associated with intentional charging and discharging of the battery, the inner surfaces of these metallic electrodes are corroded at a rate which is intended to be uniform across the surface of the metal. In addition and in contrast with this, however, these electrodes sometimes undergo an undesired and localized corrosion, the consequences of which may be reduced capacity, shorter shelf life, and a leakage of electrolyte to the outside of the battery. While this undesired local corrosion may occur randomly at any location on the surface of the electrode, experience has shown that it is particularly likely to occur at areas where electrical contact is made to the outer surface of the electrode or where the outer surface of the electrode is exposed to the air.
In an effect to reduce or eliminate the undesired corrosion that appears to occur randomly, a variety of corrosion inhibitors have been applied to the electrochemically active surface of a battery's zinc electrode in order to reduce the rate at which that surface reacts, particularly during the shelf life of the battery. For example, in U.S. Pat. No. 2,231,319 an adhesive film is shown being used between superimposed layers of thin zinc, with the adhesive being dissolved, disintegrated, decomposed or otherwise affected by the electrolyte; as a result the local couples which develop in dry cells because of process forming operations and the use of hot solder are avoided. Wood balsams and a CrO.sub.3 radical are shown in U.S. Pat. Nos. 2,279,575 and 2,343,194, respectively, as being applied onto the inner exposed surface of the zinc electrode, thereby inhibiting the corrosion which occurs during periods of battery inactivity and thereby improving the shelf life and intermittent drain properties of the battery. In all of these illustrations the corrosion inhibitor is applied over the entire electrochemically active surface of the zinc electrode, and an improvement in the shelf life of the battery is undoubtedly obtained only at the cost of some reduction in the discharge rate during intentional discharge of the battery.
As mentioned above, areas of the metallic component where electrical connections are made have been observed to be places where the undesired, localized corrosions frequently occur. While the corrosion problem may occur with connections relying upon pressure or adhesives to maintain good contact, the problem is particularly great when connections are made using heat. The heat alters the grain structure of the metal in the vicinity of the electrical connection, thereby sensitizing that portion of the metallic component to the undersired corrosion. This problem can be accelerated or aggravated by solder connections, due not only to the heat associated with soldering but also the dissimilarity of the solder metal (e.g., lead and tin) to the metallic component of the battery to which the connection is made (e.g., zinc). The corrosion problem becomes increasingly severe as the thickness of the metallic component is reduced, causing the potential corrosion and its consequences to be one of the principal limiting factors on the possible thinness of the metallic component and the battery. Increasing the thickness of the metal to combat the possible corrosion problem results in increased battery thickness and/or reduced capacity, increased costs, and reduced electrochemical efficiency where the metallic component is an electrode of the battery (e.g., a zinc anode).
Another consideration of importance in the design and manufacture of batteries is the ability to make an electrical connection to the exterior of the battery after the battery assembly process has been completed. The prior art shows electrical connections being soldered to the electrochemically active surface of a zinc anode, with the soldered connection then being coated with a deposit of electrically insulating adhesive which would prevent the solder from electrochemically "poisoning" the battery; see U.S. Pat. No. 2,870,235. In that construction, however, the solder connection and the adhesive application are made before the zinc electrode is assembled into the battery, and the problems of soldering to the exterior of a live, assembled battery are not addressed or resolved. Also as shown in U.S. Pat. No. 2,870,235, both the solder connection and the adhesive coating appear on a surface of the zinc electrode which is in contact with the battery electrolyte.
In some battery designs the metallic component may have an exterior surface which is totally or partially exposed to the air, even though no electrical connection is made to that surface as a final step in the assembly of an otherwise completed battery. Such a surface may be left intentionally exposed so that electrical contact may be made with an appliance long after the battery has been assembled, and the ability of the battery to withstand undesired corrosion during the shelf like period is highly important. Such surfaces represent areas where undesired corrosion is likely to occur and where steps to prevent it are especially needed.
There is, therefore, a need for a means to prevent localized corrosion which simultaneously meets several requirements: reduces or eliminates the undesired corrosion in those local areas where it is most likely to occur, without reducing the electrochemical activity of the remainder of the metallic surface; permits use of thin metallic components, such as thin foils that are commercially available; permits use of electrical connections, particularly heat connections, while combating the possible effects of grain restructuring and of poisoning caused by dissimilar metals; permits electrical connections to be made with heat to the exterior of previously assembled batteries; and protects where an area of the metal is to be exposed to the air for long periods of time.